1. Field of the Invention
The present invention relates to a green diamond suitable for ornamental purposes and a method of producing such a green diamond.
2. Background Art
Diamonds are classified into four types, Ia, Ib, IIa and IIb, as shown in Table 1, according to the kind and distribution of impurities contained and their optical properties.
TABLE 1 __________________________________________________________________________ Classi- Configuration in which Nitrogen Exists fication Isolated nitrogen 1 pair of 2 pairs of 3 nitrogen of Diamonds Color atoms nitrogen atoms nitrogen atoms atoms __________________________________________________________________________ IIa Transparent Absent Absent Absent Absent colorless Ia Transparent Absent Present Present Absent colorless Light Absent Present Present Few yellow Yellow Present Present Present Present Brown Many Present Present Many IIb Blue Absent Absent Absent Absent (containing B) Ib Clear Present Absent Absent Absent yellow __________________________________________________________________________
Most of the natural diamonds are of the Ia type, except that about 1% is of the IIa type, about 0.2% is of the Ib type, and the IIb type occurs rarely. On the other hand, most of the artificial diamonds produced by the temperature gradient method are of the Ib type.
Diamonds, whether natural or artificial, can be colored because of the presence of color centers in crystals. Color centers stem from lattice defects and nitrogen atoms. Typical color centers in the visible range are given in Table 2.
TABLE 2 ______________________________________ Name Composition of color center Color ______________________________________ H3 Center 1 pair of nitrogen atoms - Yellow lattice defect H4 Center 2 pairs of nitrogen atoms - Yellow lattice defect N-V Center Isolated nitrogen atoms - Purple lattice defect GR1 Center Lattice defect Bluish green ______________________________________
The characteristics and artificial production of these color centers are discussed in detail by John Walker in the Reports on Progress in Physics, Vol. 42, 1979. The artificial production method of color centers outlined therein comprises the steps of forming lattice defects in crystals by electron beam irradiation and, if necessary, performing annealing to cause said lattice defects to combine with nitrogen atoms contained in the crystals.
The color of a diamond colored by utilizing this color center formation method is the color of the rough diamond combined with the color of the color center produced. Thus, in order to obtain the ornamental value which means transparency and brilliant color, it has been usual practice to use, as rough diamonds, transparent white or light yellow, abundantly-occurring natural diamonds of the Ia and IIa types. These Ia and IIa type natural diamonds either have no nitrogen whatsoever or have one or two pairs of nitrogen atoms, as shown in Table 1. Therefore, as can be seen from Table 2, the color centers which can be formed in these rough diamonds by the method described above, are only the bluish green GR1 center (lattice defect), yellow H3 center (one pair of nitrogen atoms - lattice defect) and yellow H4 center (2 pairs of nitrogen atoms - lattice defect). Therefore, conventional colored diamonds have been limited in color to yellow and bluish green.
A report on a study of H2 centers which show green is published in The Reports on Progress in Physics mentioned above. According to this report, H2 centers are formed by a method which comprises the steps of irradiating a Ia type natural diamond with an electron beam having not less than 1.times.10.sup.16 -1.times.10.sup.18 electrons/cm.sup.2 at an accelerating voltage of 1-5 MeV and heat-treating it at a temperature of 600.degree.-1200.degree. C. in a vacuum. With this method, however, H3 and H4 centers are inevitably formed by the one pair and two pairs of nitrogen atoms simultaneously contained in the diamond. In that case, since absorption of H3 centers is stronger than that of H2 centers, the diamonds that can be obtained are yellow diamonds only. Furthermore, with this conventional method used, it has been found that if a diamond of the Ib type which does contain nitrogen but in which nitrogen atoms exist isolated, is used as a rough diamond for coloring purposes, no H2 center is formed at all; much less the use of the IIa type which contains no nitrogen.